This invention generally relates to circuit materials, methods for the manufacture of the circuit materials, and articles formed therefrom, including circuits and circuit laminates.
As used herein, a circuit material is an article used in the manufacture of circuits and multi-layer circuits, and includes circuit subassemblies, bond plies, resin coated conductive layers, unclad dielectric layers, and cover films. A circuit laminate is a type of circuit subassembly that has a conductive layer, e.g., copper, fixedly attached to a dielectric layer. Double clad circuit laminates have two conductive layers, one on each side of the dielectric layer. Patterning a conductive layer of a laminate, for example by etching, provides a circuit. Multilayer circuits comprise a plurality of conductive layers, at least one of which contains a conductive wiring pattern. Typically, multilayer circuits are formed by laminating one or more circuits together using bond plies, by building up additional layers with resin coated conductive layers that are subsequently etched, or by building up additional layers by adding unclad dielectric layers followed by additive metallization. After forming the multilayer circuit, known hole-forming and plating technologies can be used to produce useful electrical pathways between conductive layers.
A dielectric layer can comprise a polymeric dielectric composite material in which the dielectric and physical properties are controlled by the use of mineral or ceramic particulate fillers. Particularly where a low dielectric constant is desired, hollow glass or ceramic microspheres can be used. For example, U.S. Pat. No. 4,134,848 (Adicoff et al.) describes a composite for a strip line board material that contains hollow, air-filled glass microspheres in a hydrocarbon matrix. U.S. Pat. No. 4,661,301 (Okada and Fujino) discloses a hollow-glass microsphere-filled polymer composite made by directly extruding a molten composition into the opening of a vertical double belt press. U.S. Pat. No. 5,126,192 (Chellis et al.) discloses a filled prepreg material having a dielectric constant below 3.2 and made using hollow microspheres from various manufacturers. U.S. Pat. No. 4,610,495 (Landi) discloses the use of a layer of elastomer filled with hollow microspheres for controlling impedance in a solderless connector for a microelectronic device. U.S. Pat. No. 4,994,316 (Browne and Jarvis) discloses a bonding layer for circuit boards containing hollow glass microspheres.
Following these earlier patents, U.S. Pat. No. 8,187,696 (Paul et al.) disclosed, as a less costly alternative to the use of synthetic microspheres in circuit products, the use of naturally occurring hollow microspheres known as cenospheres, so long as the cenospheres meet certain compositional requirements. Selected cenospheres were found to advantageously provide a low Dk and other desired electrical properties, while maintaining the filler volume necessary for preservation of mechanical properties. Following commercial production, however, the concern developed that the available quantities might not be guaranteed indefinitely. This supply constraint coupled with the variable nature of the naturally sourced product, even on a lot to lot basis, has prompted investigation into synthetic alternatives for the use of cenospheres. A specific desire is to obtain filler having electrical properties necessary for high frequency applications that require a low dissipation factor in circuit subassemblies.
Hollow glass microspheres have been manufactured for a wide variety of uses in composite materials. For example, hollow microspheres have been used as a component of syntactic foams for the hulls of submersibles. Such microspheres have also been used for storage and/or slow release of pharmaceuticals or hydrogen gas. The microspheres typically have a diameter ranging from 10 to 300 micrometers and are sometimes termed microballoons or glass bubbles.
Hollow glass microspheres can be made by a variety of processes, including ultrasonic spray pyrolysis. The desired properties of the formed microspheres can be improved, for certain uses, by surface treatment that can involve removing at least some of the sodium. For example, among other reasons, producing microspheres having a clean surface can enhance the wettability of the microspheres by various polymers. Also, sodium depletion of the microspheres may be desirable for applications in which glass microspheres are mixed with a chemically sensitive resin. Finally, surface treatment of microspheres can improve bonding for coupling reactions if desired.
U.S. Pat. No. 4,904,293 (Garnier) discloses treatment of glass microspheres, after their production and recovery, to increase thermal resistance, by contacting the microspheres with a dealkylization agent that increases the silica content, thereby reducing the sodium content. After production, the alkaline oxide content of the microspheres is ordinarily less than 10%. In this case, the dealkylization has the aim of bringing the alkaline oxide content below 4% or less.
Leforge, J. W. et al., in “The Development of Silica Hollow Microspheres for Use as a High Temperature Dielectric,” Technical Report 60-899 (July 1961), prepared by Emerson & Cuming under USAF Contract No. AF33 (616)-7263, available from Armed Services Technical Information Agency, Arlington, Va., discloses the production of bulk microsphere materials having a low dielectric constant (less than 2.0) and a low dissipation factor (less than 0.008) that are useful at temperatures greater than 200° C. In particular, sodium borosilicate glass microspheres (“microbubbles”) were acid leached to remove sodium in order to increase high temperature stability (1090° C.). Specifically, Eccosphere® microspheres of various densities were acid leached for various periods of time in various concentration of H2SO4 or HCl. Following acid leaching of the microspheres, the authors found that a slightly lower dielectric constant and equivalent loss tangent values were obtained, i.e., the authors found that the loss tangents did not decrease in spite of removal of all extractable sodium.
In view of the above, there remains a need in the art for low dielectric constant, low loss circuit materials having improved (lower) dissipation factor.